Blocking subscribers

Remember how sometimes we have to stop the main thread from racing past an Observable or Flowable that operates on a different thread and keep it from exiting the application before it has a chance to fire? We often prevented this using Thread.sleep(), especially when we used Observable.interval(), subscribeOn(), or observeOn(). The following code shows how we did this typically and kept an Observable.interval() application alive for five seconds:

 import io.reactivex.Observable;
 import java.util.concurrent.TimeUnit;

 public class Launcher {

     public static void main(String[] args) {
         Observable.interval(1, TimeUnit.SECONDS)
                 .take(5)
                 .subscribe(System.out::println);

         sleep(5000);
     }

     public static void sleep(int millis) {
         try {
             Thread.sleep(millis);
         } catch (InterruptedException...